This invention relates to photosensitive compositions and to photosensitive elements, for example, printing plates, embodying a layer of such compositions. More particularly, the invention relates to a process for making lithographic plates.
Compositions capable of being converted under the influence of radiation to rigid, insoluble, tough structures have become increasingly important in the preparation of printing elements. One of the fundamental patents relating to such compositions in U.S. Pat. No. 2,760,863 to Plambeck. In the process of the Plambeck patent, printing elements are produced directly by exposing to actinic light, through an image bearing process transparency, a layer of an essentially transparent composition containing an addition polymerizable, ethylenically unsaturated monomer and an addition polymerization initiator activatable by actinic radiation. The layer of polymerizable composition is supported on a suitable support, and exposure of the composition is continued until substantial polymerization of the composition has occurred in the exposed areas with substantially no polymerization occurring in the nonexposed areas. The unchanged material in the latter areas then is removed, as by treatment with a suitable solvent in which the polymerized composition in the exposed areas is insoluble. In the case of printing plates, this results in a raised relief image which corresponds to the transparent image of the transparency and which is suitble for use in letterpress work.
While extremely useful in the preparation of relief printing elements and images from dry transfer processes, the photopolymer compositions of the types disclosed by the Plambeck patent become less sensitive to radiation due to the diffusion of oxygen from the air into the photopolymer layer. The oxygen acts to inhibit the desired polymerization and crosslinking reactions. There are means of removing or preventing oxygen from saturating or desensitizing the photopolymer layer. One way is to store or treat the element in an essentially oxygen-free atmosphere of an inert gas such as carbon dioxide. This technique gives satisfactory results but requires special equipment and is time consuming. It also is known to add certain metal compounds such as tin salts, which are soluble in the photopolymer composition but which are nonreactive with the addition polymerization initiator. While a number of these compounds substantially reduce the influence of oxygen and improve the photograhic speed of the photopolymer element, their utilization has not been entirely satisfactory.
U.S. Pat. Nos. 4,271,259 and 4,272,610 disclose a photochemical process for the preparation of printing plates that is not inhibited by oxygen and in fact depends upon oxygen being present during the exposure step. During exposure the ambient, triplet oxygen is converted to singlet oxygen which is involved in the formation of hydroperoxides and peroxides. These intermediates are subsequently decomposed either concurrently with or subsequent to their formation, preferably in the absence of oxygen, thereby generating free radicals which effect polymerization and/or crosslinking of the photosensitive composition. The process includes the steps of (A) providing a photosensitive composition in film form comprising (1) a photooxygenation sensitizer, (2) an ethylenically unsaturated component capable of forming a high polymer by addition polymerization or crosslinking and (3) a photooxidizable component containing olefinic unsaturation of the type in which there is no more than one hydrogen atom on each of the double bond carbons and in which there is at least one allylic hydrogen on at least one of the carbon adjacent to the double bond carbons, which allylic hydrogen is not on a bridgehead carbon, (B) exposing selected areas of the sensitized film to light having a wave length of from about 2000 to about 12000 Angstroms in the presence of oxygen and (C) treating the exposed film with heat, a metal catalyst or a nonmetallic reducing agent to form a crosslinked polymer in the exposed areas of the film.